1. Field of the Invention
This invention relates to a multi-beam scanning optical system using a light source having a plurality of light emitting sections such as a semiconductor laser array and adapted to high speed high density recording. A multi-beam scanning optical system according to the invention can suitably be used for an image forming apparatus such as a laser beam printer or a digital copying machine.
2. Related Background Art
Recently, there is a growing demand for image forming apparatus such as laser beam printer and digital copying machines that can produce images at high speed with an enhanced level of resolution. Multi-beam scanning optical systems using a light source having a plurality of light emitting sections such as a semiconductor laser array have been proposed to meet the demand.
FIG. 1 of the accompanying drawings is a schematic cross sectional view of a known multi-beam scanning optical system taken along the main-scanning direction. Referring to FIG. 1, a plurality of light beams emitted from so many semiconductor lasers (or a semiconductor laser array) 91 are substantially collimated by a collimator lens 92 and then converged only in the sub-scanning direction by a cylindrical lens 94 having a predetermined refractive power only in the sub-scanning direction. The light beams are then trimmed by an aperture stop 93 and focussed on or near a deflecting plane (reflecting plane) 95a of an optical deflector 95, which is a rotary polygon mirror, to produce a substantially linear image extending in the main-scanning direction. Then, the light beams deflected/reflected by the deflecting plane 95a of the polygon mirror 95 that is rotating at a predetermined angular velocity in the sense of arrow 95b in FIG. 1 are focussed on the surface of a photosensitive drum 97 to be scanned as so many spots by way of a pair of f.theta. lenses 96a and 96b of a scanning optical system 96 and made to optically scan the surface of the photosensitive drum 97 in the direction of arrow 97b in FIG. 1.
FIGS. 2 and 3 are schematic illustrations of the arrangement of light emitting sections of the light source of a multi-beam scanning optical system. In FIGS. 2 and 3, reference symbols A and B denote respective light emitting sections and reference symbol M denote the main-scanning direction whereas reference symbol S denotes the sub-scanning direction.
In a multi-beam scanning optical system having a configuration as described above, if a plurality of light emitting sections A and B are arranged longitudinally along the sub-scanning direction as shown in FIG. 2, the scanning lines on the surface of the photosensitive drum produce large intervals in the sub-scanning direction that are by far greater than the values required for achieving a desired recording density. To avoid this problem, the light emitting sections A and B are arranged on a line that forms a non-zero angle .delta. with the sub-scanning direction (S) as shown in FIG. 3. Thus, the intervals separating the scanning lines on the surface of the photosensitive drum can be accurately regulated to achieve the desired recording density by selecting an appropriate value for angle .delta.. In other words, the light emitting sections of the light source are arranged at respective positions that vary from each other in terms of both the main-scanning direction (M) and the sub-scanning direction (S) that is perpendicular relative to the main-scanning direction (M).
Meanwhile, if the light beams striking the surface of the photosensitive drum are regularly reflected by it and returned to the semiconductor lasers, the oscillations of the semiconductor lasers can become unstable. Additionally, if the regularly reflected light beams are returned to the optical system, they can be reflected once again by the surface of the optical system to irradiate the surface of the photosensitive drum and give rise to a ghost there. To avoid these problems, the angle (of incidence) .alpha. between the light beams striking the surface 97 of the photosensitive drum and the normal to the surface 97 of the photosensitive drum is made equal to a predetermined value in the sub-scanning direction in known multi-beam scanning optical systems as shown in FIG. 4. With this arrangement, the light beams emitted from the semiconductor lasers are not regularly reflected by the surface 97 of the photosensitive drum and returned to the optical system. FIG. 4 is a schematic cross sectional view of a known multi-beam scanning optical system taken along the sub-scanning direction. Note that, in FIG. 4, the components same as those of FIG. 1 are denoted respectively by the same reference symbols and will not be described any further.
Then, as shown in FIGS. 5A and 5B, the plurality of scanning lines on the surface 97 of the photosensitive drum by a multi-beam scanning optical system having a configuration as described above show different scanning magnifications as illustrated in FIGS. 5A and 5B. As a result, the spots formed by focussed beams of light on the surface 97 of the photosensitive drum show a displacement (D) in the main-scanning direction to make it no longer possible to produce a high quality image. Note that FIGS. 5A and 5B are schematic cross sectional view of a principal portion of a known multi-beam scanning optical system and a photosensitive drum shown to illustrate the problem thereof. In FIGS. 5A and 5B, reference symbols 95 and 96 respectively denote a polygon mirror and a scanning lens system and reference symbol D denotes the displacement of beam spots.
A number of proposals have been made to dissolve the above identified problem, including those disclosed in Japanese Patent Applications Laid-Open Nos. 5-333281 and 9-197308. Japanese Patent Application Laid-Open No. 5-333281 describes an arrangement for reducing the displacement of focal point in the main-scanning direction by making the angle between the light beams striking the surface of the photosensitive drum and the normal to the surface of the photosensitive drum in the sub-scanning direction smaller than a predetermined value. On the other hand, Japanese Patent Application Laid-Open No. 9-197308 describes an arrangement for reducing the displacement of focal point in the main-scanning direction by arranging the imaging optical system eccentrically and regulating the extent of the eccentricity.
However, none of the proposed multi-beam scanning optical systems including those described in the above cited patent documents can satisfy both the requirement of reducing the displacement of focal point in the main-scanning direction and that of forming an excellent (spot-shaped) image.
More specifically, as pointed out, Japanese Patent Application Laid-Open No. 5-333281 describes an arrangement of making the angle between the light beams striking the surface of the photosensitive drum and the normal to the surface of the photosensitive drum in the sub-scanning direction smaller than a predetermined value. However, this arrangement can only reduce the displacement of focal point in the main-scanning direction to make it less remarkable. In other words, the method proposed in Japanese Patent Application Laid-Open No. 5-333281 cannot dissolve the problem of displacement of focal point in the main-scanning direction.
On the other hand, Japanese Patent Application Laid-Open No. 9-197308 describes an arrangement for reducing the displacement of focal point in the main-scanning direction by arranging the imaging optical system eccentrically and regulating the extent of the eccentricity. However, when the imaging optical system is arranged eccentrically, the spot-shaped image formed on the surface of the photosensitive drum can easily become deformed to make it difficult to realize a high quality and high speed image recording.